1. Field of the Invention
This invention relates to circuit interrupters including arc fault trip mechanisms and, more particularly, to electronic trip units for circuit breakers, which respond to sputtering arc faults. The invention also relates to apparatus for detecting arc faults.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bi-metal, which is heated and bends in response to a persistent overcurrent condition. The bi-metal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
There has been considerable interest in providing protection against arc faults. Arc faults are intermittent high impedance faults which can be caused, for instance, by worn insulation between adjacent conductors, by exposed ends between broken conductors, by faulty connections, and in other situations where conducting elements are in close proximity. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average RMS current to trip the conventional circuit interrupter. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads, which draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for responding to arc faults. See, for example, U.S. Pat. Nos. 5,224,006; 5,691,869; and 5,818,237.
It is believed that most arc fault detectors perform poorly or are completely ineffective in detecting wet track arc faults in wiring harnesses comprised of polyimide (e.g., Kapton®) insulated wire because such faults exhibit nearly sinusoidal fault currents. It is believed that known arc fault detectors do not respond unless the arc fault current exhibits a relatively more severely distorted characteristic.
Some known prior art arc fault detectors for aerospace applications trip on relatively high, near sinusoidal currents and sinusoidal currents, which are caused by wet track arc faults and by motor inrush, respectively. For example, if the absolute current amplitude exceeds a predetermined threshold (e.g., about five times rated current), then the arc fault detector initiates a trip. There is a need for an arc fault detector to eliminate nuisance tripping that results from an inability to discern between those two types of currents.
Accordingly, there is room for improvement in circuit breakers and apparatus for detecting arc faults.